Patent Document 1 has described such a problem that burrs will occur easily upon cutting these difficult-to-cut materials such as stainless steel, these burrs undergo work hardening to result in extremely low machinability and, therefore, a cutting edge of a cutting boundary which is exposed to the burrs is increased in chipping to fracture the cutting edge or greatly decrease the tool life due to what is called abnormal wear.
Therefore, Patent Document 1 proposes a cutting insert in which a cutting edge is formed substantially linearly at an inclination angle which is greater than 0° but smaller than 20° so that a distance of the cutting edge from a bottom face of a flat insert main body is small at a corner section and gradually increases as moving toward the center of the cutting edge, a breaker rake face of a chip breaker on an upper face is set at an angle of 20 to 30° and there is formed at an inner part of the breaker rake face a breaker wall which rises in a direction at which the height of the bottom face is gradually increased from a bottom of the chip breaker toward the inner part thereof, by which an upper face part inside the chip breaker is made lower than a peripheral cutting edge section.
According to the above-described cutting insert, it is possible to reduce the occurrence of burrs even where the cutting edge is increased in inclination angle to cut a difficult-to-cut material such as stainless steel. Further, abnormal wear can be suppressed as much as possible to extend the tool life. Furthermore, what is called tenacious chips coming from a difficult-to-cut material can be broken into pieces reliably by the breaker wall of the chip breaker. Still further, even where the chips move along the breaker wall, resistance associated with the movement thereof can be reduced to prevent heating.
Further, Patent Document 2 proposes a cutting insert which is used appropriately in cutting stainless steel, that is, a breaker flute is formed to have a cross section which is constituted with a land (rake angle θ1), a first inclination face (rake angle θ2), a second inclination face (rake angle θ3) and a steep face, each of which has a positive rake angle sequentially from a cutting edge to a center so as to give the relationship of θ2>θ1≧θ3>0, and the rake angle θ1 of the land is increased at the corner but decreased at the center of a major cutting edge, while the width of the land is decreased at the corner but increased at the center of the major cutting edge.
According to Patent Document 2, since the contact area between the breaker flute and the chips is small, the thus constituted cutting insert can suppress cutting resistance. Further, since a position of the cutting edge corresponding to the depth of cut where fracturing will take place easily is strengthened, the position is to be high in fracturing resistance. Next, Patent Document 2 describes that the above-described result has been obtained in particular when the thus constituted cutting insert is used to perform light cutting of stainless steel (SUS304) under conditions that the cutting speed is 100 m/min, the depth of cut is 1 mm and the feed rate is 0.2 mm/rev.